Line-driver with power down loopback protection

ABSTRACT

A line driver selectively drives one of two transmission lines. The line driver includes a differential amplifier connected to first and second differential switches. The first differential switch is connected between an output of the differential amplifier and a first of two transmission lines. The second differential switch is connected to the output of the differential amplifier and to the second of two transmission lines. The first and second differential switches are controlled by respective first and second control signals. The output of the differential amplifier is connected to either the first or the second transmission line in response to the first and second control signals. The differential switches include loopback protection to an prevent an incoming signal from passing from one transmission line to another during power down mode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims benefit of U.S. Provisional Application No.60/243,483, filed Oct. 26, 2000, which is incorporated by referenceherein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to computer connections in alocal area network, and more particularly to node devices having MediaDependent Interfaces (MDI) that are installed in such networks and theirautomatic connection configuration.

2. Background Art

Many local area network products (LAN) use a medium consisting oftwisted copper wire pairs for the transmission and reception of data. Ata LAN node, one or more twisted pairs transmit data to a second LANnode, and one or more twisted pairs receive data from the second LANnode. This arrangement permits simultaneous data transmission andreception, also called full duplex communications.

In a conventional LAN node with full duplex communications, differenttwisted pairs are used for transmission and reception. This requiresthat each end of the link have a defined assignment for its twisted pairconnector. For example, a Network Interface Card (NIC) is usuallyemployed as an end node device, while a switch or a repeater will beemployed as a concentrator or central component in a star-based network.During link operation, the NIC transmits data on a pair of wires thatare connected to the receiver of the hub or switch, and the NIC receivesdata on a pair of wires that are connected to the transmitter of the hubor switch. If the NIC transmit pair of wires are inadvertently connectedto the hub transmit connector, then the communication link will fail.Similarly, if the NIC receive transmit pair of wires are inadvertentlyconnected to the hub receive connector, then the communications linkwill also fail.

Most LAN standards assign specific connector pins to the wires in thetwisted pair to prevent the transmit and receive twisted pairs frombeing crossed at one of the nodes. In the IEEE-802.3 10BASE-T standard,pins 1 and 2 at an end node are connected to the transmit twisted pair,and pins 3 and 6 are connected to the twisted receive pair. At the hub,which is typically a repeater or a switch for a 10BASE-T network, pins 3and 6 are connected to the transmit pair, and pins 1 and 2 are connectedto the receive pair. This works quite well for configurations whereNIC's are attached to repeaters. However, other configurations arepossible with the emergence of switched networks. For example, two NICscan be directly connected together, or two repeaters can be directlyconnected together, or a repeater can be connected to a switch.Depending upon the pin assignment of these devices, it may becomenecessary to employ a crossover cable to connect two LAN devices thathave incompatible pin assignments.

A manual switch can be employed to switch the connections for thetransmit and receive pairs for a hub, repeater, or switch. The switchallows the products to connect to other similar devices but requires aninstaller to manually push a button. The manual approach works well forlimited applications like a repeater-to-repeater links, but not in thegeneral case where it is desired to build a LAN device that attaches toeither a repeater, a NIC, or a switch, without manual intervention.

What is needed is an automated means of switching transmitter orreceiver connections for a generalized LAN device, including a hub,repeater, or a NIC.

BRIEF SUMMARY OF THE INVENTION

The present invention includes a line driver that is used to selectivelydrive one of two transmission lines. The line driver is a differentialamplifier connected to first and second differential switches. The firstdifferential switch is connected between an output of the differentialamplifier and a first of two transmission lines. The second differentialswitch is connected to the output of the differential amplifier and tothe second of two transmission lines. The first and second differentialswitches are controlled by respective first and second control signals.The output of the differential amplifier is connected to either thefirst or the second transmission line in response to the first andsecond control signals.

Power down loopback protection is provided by passive devices thatprevent an external network signal from feeding through the unpoweredline driver differential switches and injecting a false signal into thenetwork.

An advantage of the present invention is more efficient use of valuableintegrated circuit space by using a line driver in place of two linedrivers.

Another advantage is the use of a single differential amplifier to driveboth output transmission lines. This feature greatly reduces oreliminates the need for post assembly line balancing required when twoindependent line drivers are used.

A further advantage is the excellent signal isolation achieved bycoupling a signal to the input of two parallel switches, thenmaintaining one switch closed while opening the other.

Another advantage is the mitigation of power down loopback signalswithout adding additional circuitry or using additional chip space. Theprotection is passive and does not consume any additional power.

Further features and advantages of the present invention, as well as thestructure and operation of various embodiments of the present inventionare described in detail below.

BRIEF DESCRIPTION OF THE FIGURES

The present invention is described with reference to the accompanyingdrawings. In the drawings, like reference numbers indicate identical orfunctionally similar elements. Additionally, the leftmost digit(s) of areference number identifies the drawing in which the reference numberfirst appears.

FIG. 1. illustrates an example environment for the invention;

FIG. 2. illustrates a conventional Auto-MDIX circuit;

FIG. 3. illustrates a single line-driver Auto-MDIX circuit;

FIG. 4. illustrates a power down loopback transient;

FIG. 5. illustrates details of power down loopback protection;

FIG. 6. illustrates an Auto-MDIX line-driver with power down loopbackprotection.

DETAILED DESCRIPTION OF THE INVENTION

The present invention generally relates to an apparatus for configuringmedia connections in a local area network. In one embodiment, thepresent invention is particularly directed to configuring mediaconnections for use with twisted copper wire pairs. However, other mediaconnections could also be used as will be further described herein. Inone embodiment, the invention is particularly suited for local areanetworks that operate on a 10Base-T standard, a 100Base-T standard, aswell a 1000Base-T standard. The invention is also suited for TP-PMD,Token Ring, and others.

Before describing the invention in detail, it is useful to describe anexample environment for the invention. FIG. 1 is a block diagramillustrating an example auto-MDIX environment 100 comprising a clientcomputer 102, a network 104, a plurality of network devices 106, anetwork interface card 110, an Auto-MDIX circuit 112, a firsttransmission line 114, a second transmission line 115, a client computeroperating system 120, a network protocol interpreter 118, and a networkinterface card driver 116.

The network interface card (NIC) 110 links the client computer 102 withthe network 104. More specifically, the NIC 110 can link the computer102 to a specific network device 106, or to a group of network machines,or to the network in general (i.e. a query). To exchange data with thenetwork devices 106, the data must be placed in a compatible format forexchange over a network medium and then physically sent over thatmedium. The client operating system 120 writes a data request 119 tousing a client information request to the network 104. The operatingsystem 120 sends the data request 119 to the network protocolinterpreter 118. The network protocol interpreter 118 identifies thetarget network and receives any instructions from the operating system120 that are to be provided to the NIC 112. Based on information fromthe operating system 120, the network protocol interpreter 118 generatesa data request 117 that in the proper format to be addresses,transmitted, routed, and acted upon by the network 104.

The NIC driver 116 receives network data and determines the transmissionmedium that is to be utilized. The NIC driver 116 generates specificinstructions for the NIC 110 based on transmission medium that is to beutilized. In this example, then NIC 110 uses two transmission lines 114and 115 to control the transmission of transmit data 124. The NIC driver116 sends transmission control signals 122 and 123 to control thetransmission of the transmit data 124 on the transmission lines 114 and115, respectively.

After the data is transmitted, the NIC 110 listens for returning data,either from an unrelated event or an answer to a query from theoperating system 120. The NIC 110 generates a receiver control signal128 to ensure the receiver (not shown) is lined up to the correcttransmission line and directs the received data 126 to a storagelocation in the client computer.

The control of receiving and transmitting from the NIC 110 is veryimportant. Twisted pair lines use the same physical media fortransmitting and receiving. Collisions of data on a network can causedata loss and possibly reduce network stability.

FIG. 2 illustrates a conventional auto-MDIX circuit 112. Auto-MDIX 112comprises a first line driver 204, a second line driver 202, a receiver206, the receiver control signal 128, the first transmit control signal122, the second transmit control signal 123, the first transmission line114, and the second transmission line 115.

Transmit data 124 is placed at the input to the first line driver 204and the second line driver 202. When the first transmit control signal122 is received, the first line driver 204 transmits data to the firsttwisted wire pair 114. When the second transmit control signal 123 isreceived, the second line driver 202 transmits data to the second wirepair 115. If the first line driver 204 is transmitting over the firsttransmission line 114, the receiver control 128 can direct data from thesecond transmission line 115 to the receiver 206. Alternatively, if thesecond line driver 202 is transmitting over the second transmission line115, the receiver control 128 can direct data from the firsttransmission line 114 to the receiver 206.

In one embodiment, the invention is implemented using field effecttransistors. For simplicity, in the remaining disclosure theabbreviation FET is used to indicate a field effect transistor was usedas the component described. One of skill in the relevant art willunderstand the invention is not limited to implementations having fieldeffect transistors.

FIG. 3 illustrates a line driver 300 according to embodiments of theinvention. The line driver 300 has the ability to drive either the firsttransmission line 114 or the second transmission line 115 based on thecontrol signals 122 and 123, without the need for a second line driver.The line driver 300 includes a differential amplifier 312 coupled to afirst differential switch 313 and a second differential switch 314. Thefirst control signal 122 controls the first differential switch 313, andthe second control signal 123 controls the second differential switch314. A media interface 316 is coupled to the first and seconddifferential switches 313 and 314.

The media interface 316 provides the electrical and mechanical adaptorsnecessary to couple a desired media with the output of the first andsecond differential switches 313 and 314. In the embodiment shown inFIG. 3, the transmission lines 114 and 115 are two twisted pairconductors, and the media interface includes a first media coupler 350and a second media coupler 352, both of which are center tappedtransformers. The first media coupler 350 connects the output of thefirst differential switch 313 to the transmission line 114 and thesecond differential switch 314 connects the output of the seconddifferential switch 314 to the transmission line 115. The presentinvention is not limited to twisted pair conductors. Other possibletransmission media includes coaxial cable, fiber optic cable, shieldedtwisted pair, unshielded twisted pair, and other transmission media thatwill be understood by those skilled in the arts based on the discussiongiven herein.

The differential amplifier 312 includes a first FET 332, a second FET328, and a third FET 334 that are configured as shown. The first FET 332provides a bias current for the second FET 328 and the third FET 334,where the bias current is determined by a bias voltage 335 that controlsthe gate of the first FET 332. The gate of the second FET 328 receives afirst component 124 a of the differential input signal 124, and the gateof the third FET 334 receives a second component 124 b of thedifferential input signal 124. The second FET 328 and the third FET 334amplify the differential input signal 124 according to the bias currentprovided by the FET 332. The resulting differential output signal 327(having components 327 a and 327 b) is provided to both the firstdifferential switch circuit 313 and the second differential switchcircuit 314.

The differential amplifier 312 is only one circuit configuration for adifferential amplifier. Other equivalent differential amplifiers,including other current mode and voltage mode differential amplifiers,could be utilized in the line driver 300. These other differentialamplifier configurations are within the scope and spirit of the presentinvention.

The first differential switch 313 switches the differential outputsignal 327 to the first transmission line 114 when the firstdifferential switch 313 is activated by the control signal 122.Likewise, the second differential switch 314 switches the differentialoutput 327 to the second transmission line 115 when the seconddifferential switch 314 is activated by the control signal 123,respectively. Further details of the first and second differentialswitches 313 and 314 are discussed below.

The first differential switch 313 includes a first FET switch 322 and asecond FET switch 326. The source of the first FET switch 322 receivesthe differential output signal component 327 a, and the source of thesecond FET switch 326 receives the differential output signal component327 b. The gates of the first FET switch 322 and the second FET switch326 are controlled by the first control signal 122. The drains of thefirst FET switch 322 and the second FET switch 326 are connected to thefirst transmission line 114 through the transformer 350.

The second differential switch 314 includes a third FET switch 320 and afourth FET switch 324. Similar to the first differential switch 313, thesource of the third FET switch 320 also receives the differential outputsignal component 327 a, and the source of the fourth FET switch 324receives the differential output signal component 327 b. The gates ofthe third FET switch 320 and the second FET switch 324 are controlled bythe second control signal 123. The drains of the third FET switch 320and the fourth FET switch 324 are connected to the second transmissionline 115 through the transformer 352.

When the client computer 102 is powered-up, transmit control circuitry(not shown) generates the first control signal 122 and the secondcontrol signal 123 to select either the transmission line 114 or thetransmission line 115 as the active transmission link. Alternatively,neither transmission line can be utilized.

To select the transmission line 114 as the transmission link, the firstcontrol signal 122 raises the gate voltage on the first FET switch 322and the second FET switch 326. The raised gate voltages cause the FETs322 and 326 to conduct and pass the differential amplifier output signal127 to the first transmission line 114 for transmission. In embodiments,the control signal 122 is adjusted to cascode FETs 322 and 326 with thedifferential amplifier 312 to enhance output impedance, improveisolation, and improve overall circuit performance. Furthermore, thesecond control signal 123 cuts off the third FET 320 and the fourth FET324, blocking the differential amplifier output signal 127 from thesecond transmission line 115.

To select the transmission line 115 as the transmission link, the firstcontrol signal 122 lowers the gate voltage on the first FET 322 and thesecond FET 326 so that FETs 322 and 326 are cutoff, thereby blocking thedifferential output signal 127 from the first transmission line 114.Furthermore, the second control signal 123 raises the gate voltage onthe third FET 320 and the fourth FET 324. The raised gate voltages causethe FETs 320 and 324 conduct and pass the differential output signal 127to the second transmission line 115. In embodiments, the control signal123 is adjusted to cascode FETs 320 and 324 with the differentialamplifier 312 to enhance output impedance, improve isolation, andimprove overall circuit performance.

When no signal transmission is desired, then the first control signal122 lowers the gate voltage on the first FET 322 and second FET 326.Therefore, the FETs 322 and 326 are cutoff, and the differential outputsignal 127 is blocked from the transmission line 114. Furthermore, thesecond control signal 123 also lowers the gate voltage on the third FET320 and the fourth FET 324. Therefore, the FETs 320 and 324 are cutoff,and the differential output 127 is blocked from the transmission pair115.

In embodiments of the invention, there are more than two transmissionlines that can be selected from. As such, the differential switches 313and 314 can be stepped and repeated any number of times to switchbetween the multiple transmission lines. In these stepped embodiments,the differential amplifier 112 is stepped and repeated as necessary toprovide the desired output voltage and waveshape.

In other embodiments, the line driver 300 can be a cell in a array ofcells that make up a larger line driver. Each individual line drivercell can then be controlled to individually provide the appropriateoutput voltage amplitude and waveshape.

Signal Pass Through and Loopback

The network 104 is active twenty-four hours a day and most user sessionslast just a fraction of that time. When a user is finished with theclient computer 102, it is shut down and powered off. In many clientcomputer 102 installations the network media is difficult to disconnect,and therefore is left connected to the network interface card 110.

FIG. 4 is a signal switching section 400 that is associated with thesignal line driver 300. The signal switching section 400 includes asubset of those elements in the line driver 300 that are needed toillustrate signal feed through and loopback. The elements of the signalswitching section 400 are connected identically as in the line driver300, but the circuit layout is presented to conveniently illustratesignal feedback and loopback that can occur between the firsttransmission line 114 and the second transmission 115 in the single lineloop driver 300.

Referring to FIG. 4, two signal paths exist between the firsttransmission line 114 and the second transmission line 115. The firstpath is between the second transmission line 115, a receive datapositive (RDP) node, the third FET switch 320, the first FET switch 322,a transmit data positive (TDP) node, and the first transmission line114. The second path is between the second transmission line 115, areceive data negative (RDN) node, the fourth FET switch 324, the thirdFET switch 326, a transmit data negative (TDN), and the firsttransmission line 114.

The first network transmission line 114 and the second networktransmission line 115 can carry data to the client computer 102 or carrydata from the network 104 to the client computer 102. Inadvertently,connecting two independent transmission media is undesirable and cancause data corruption, data loss, and reduced network 104 stability.

When power is applied to the line driver 300, the first FET switch 322,the second FET switch 326, the third FET switch 320, and the fourth FETswitch 324 are biased open or closed by one of two control signals 122or 123. In each transmit mode, at least one FET switch between the RDPnode and the TDP node is biased closed to prevent a signal from flowingbetween the first and second transmission lines. Likewise, at least oneFET switch between the RDN node and the TDN node is also biased closedto prevent a signal from flowing between the first transmission line 114and the second transmission line 115.

When power is removed from the line driver 300, the voltage of eachcircuit node falls to ground or floating near ground. A node floats nearground as a result of parasitic coupling or leakage currents from adevice that was previously energized.

Referring to FIG. 4, a differential link pulse 402 can be received fromthe network at the signal switching section 400. For example, thedifferential link pulse 402 comprises a positive link pulse 402 a thatis applied at RDP, and a negative link pulse 402 b that is applied atRDN. The positive voltage of the positive link pulse 402 a raises thedrain voltage of the third FET 320 relative to the gate voltage, andtherefore opens the third FET switch 320 so that the positive link pulse402 a is blocked.

The negative link pulse 402 a, applied at RDN, pulls the fourth FETswitch 324 drain below the floating gate, causing the fourth FET switch324 to conduct and pass the negative link pulse 402 b to the source ofthe second FET switch 326. The negative link pulse 402 b drops thevoltage on the source of the second FET switch 326 below the floatinggate, causing the fourth FET 326 to conduct and pass the negative linkimpulse 402 b to the TDN. The negative link pulse 402 b is applied tothe first media coupler 350 and induces a positive loopback pulse 410 atthe TDP node. The negative link pulse 402 b and the positive loopbackvoltage 410 are passed through the media coupler 350 and are transmittedas a differential false pulse 404 (having components 404 a and 404 b) onthe first transmission line 114. These false pulses 404 are undesirableas they can collide with legitimate data on the network. Also, thesefalse pulses 404 can appear as (unintentional) valid data packets beingsent back to the hub or source.

A negative incoming signal (e.g., 402 b) can arrive at one or more ofthe nodes RDP, RDN, TDP, or TDN, depending on how accurately the networkcabling is installed. Power down loopback protection can prevent thenegative signals 402 b from producing false signals 402, and isdescribed in the following section.

Power Down Loopback Protection

FIG. 5 illustrates a modified signal switching section 500, which is thesignal switching section 400 modified with loop back protection. Morespecifically, the modified signal switching section 500 includes theswitching section 400 and a first loopback FET 502, a second loopbackFET 504, a third loopback FET 506, and a fourth loopback FET 508. (Themedia transformers 350 and the 352 are not shown for convenience ofillustration.)

The loopback FET 502-508 are configured as follows. The gates of all theloopback FETs 502-508 are coupled to a ground 510. The source of thefirst loopback FET 502 is coupled to the gate of the first FET switch322, and the drain of the first loopback FET 502 is coupled to the drainof the first FET switch 322. The source of the second loopback FET 504is coupled to the gate of the second FET switch 326, and the drain ofthe second FET switch 326 is coupled to the drain of the second loopbackFET 504. The source of the third loopback FET 506 is coupled to the gateof the third FET switch 320, and the drain of the third loopback FET 506is coupled to the drain of the third FET switch 320. The source of thefourth loopback FET 508 is coupled to the gate of the fourth FET switch324, and the drain of the fourth loopback FET 508 is coupled to thedrain of the fourth FET switch 324.

Since the gate of each loopback FET 502-508 is grounded, each loopbackFET will conduct if the source or drain voltage drops below thetransistor threshold voltage V_(T). When a loopback FET conducts, itpulls the associated FET switch gate down, following the negative inputvoltage. By forcing the gate of the FET switch to follow the negativeinput voltage, the FET switch is always off and will not feed throughthe negative input pulse.

For example, if the negative voltage 402 b is applied to RDN in FIG. 5,the loopback FET 508 will conduct and pass the negative voltage 402 b tothe gate of the fourth FET switch 324. As a result, the negative voltage402 b will be applied to both the source and the gate of the switch FET324. Therefore, the source and the gate of the FET 324 will be atapproximately the same potential, and the FET 324 will not conduct.Therefore, the negative voltage 402 b will be blocked from the secondswitch FET 326, and will not produce a false pulse on the transmissionline 114. The other loopback FETs 502-506 operate in a similar mannerwhen a negative pulse 402 b is applied to particular drain nodes of theloopback FETs 502-508. Thus, the other loopback FET 502, 504, and 506provide similar loopback protection for their respective switching FETs322, 326, and 320.

When the modified signal switching section 500 is powered up, the gateof each loopback FET 502-508 is grounded. The source and the drain ofeach loopback FET are biased above ground potential, so the loopback FETis cutoff and has no effect on the respective FET switch 332-326 duringnormal powered up operations.

FIG. 6 illustrates a modified line driver 600. The modified line driver600 is the single driver 300 with the loopback protection FETs 502-508added to the respective switching FETs 302-326, to create modifieddifferential switches 602 and 604.

In the modified differential switch 602, the gates of the first andsecond loopback FETs 502, 504 are coupled to the ground 510. The sourcesof the first and second loopback FETs 502, 504 are coupled to the gateof the first FET switch 322 and the second FET switch 326, respectively.The drains of the first and second loopback FETs 502, 504 are coupled tothe drains of the first FET switch 322 and the second FET switch 326,respectively.

In the modified differential switch 604, the gates of the third andfourth loopback FETs 506, 508 are coupled to the ground 510. The sourcesof the third and fourth loopback FETs 506, 508 are coupled to the gatesof the third FET switch 320 and the fourth FET switch 324, respectively.The drains of the third and fourth loopback FETs 506, 508 are coupled tothe drains of the third FET switch 320 and the fourth FET switch 324,respectively.

As discussed in relation to FIG. 5, a negative pulse at RDN will causeloopback FET 508 to conduct so that the drain and the gate of switchingFET 324 are at the same potential. Therefore, the switching FET 324 willnot conduct and the negative going pulse will be blocked from the TDNnode, preventing a retransmission over the transmission line 115. Theother loopback FET 502, 504, and 506 provide similar loopback protectionfor their respective switching FETs 322, 326, and 320. Therefore, nonegative incoming signals on either transmission line 114 or 115 will beretransmitted on the other transmission line.

In embodiments of the invention, there are more than two transmissionlines that can be selected from. As such, the differential switches 602and 604 can be stepped and repeated any number of times to switchbetween the multiple transmission lines. In these stepped embodiments,the differential amplifier 112 is stepped and repeated as necessary toprovide the desired output voltage and waveshape.

In other embodiments, the line driver 600 can be a cell in an array ofcells that make up a larger line driver. Each line driver cell can havethe loopback protection FETs (e.g. 506, 502, 508, and 504) as shown inFIG. 6. Alternatively, since the cells are arrayed in parallel, only oneset of loopback protection FETs can be connected to the appropriate FETsin the multiple cells.

CONCLUSION

Example embodiments of the methods, circuits, and components of thepresent invention have been described herein. As noted elsewhere, theseexample embodiments have been described for illustrative purposes only,and are not limiting. Other embodiments are possible and are covered bythe invention. Such embodiments will be apparent to persons skilled inthe relevant art(s) based on the teachings contained herein. Thus, thebreadth and scope of the present invention should not be limited by anyof the above-described exemplary embodiments, but should be defined onlyin accordance with the following claims and their equivalents.

What is claimed is:
 1. A line driver, for driving one of twotransmission lines, comprising: a differential amplifier that receivesan input signal; a first differential switch having an input coupled toan output of said differential amplifier and an output coupled to afirst of said two transmission lines; a second differential switchhaving an input coupled to said output of said differential amplifierand an output coupled to a second of said two transmission lines; aloopback prevention circuit configured to prevent at least one of saidfirst differential switch and said second differential switch fromconducting during a power down mode; said first and second differentialswitches controlled by respective first and second control signals; andwherein an output of said differential amplifier is coupled to one ofsaid first and second transmission lines using said first and secondcontrol signals.
 2. The line driver of claim 1, wherein said firsttransmission line is coupled to said output of said differentialamplifier by closing said first differential switch and opening saidsecond differential switch using said respective first and secondcontrol signals.
 3. The line driver of claim 1, wherein said secondtransmission line is coupled to said output of said differentialamplifier by opening said first differential switch and closing saidsecond differential switch using said respective first and secondcontrol signals.
 4. The line driver of claim 1, wherein said firstdifferential switch comprises: a first FET coupled between said outputof said differential amplifier and a first terminal of said firsttransmission line; and wherein said loopback prevention circuit includesa second FET having a grounded gate, a source of said second FET coupledto a gate of said first FET, a drain of said second FET coupled to saidfirst terminal of said transmission line.
 5. The line driver of claim 4,wherein said first differential switch further comprises: a third FETcoupled between said output of said differential amplifier and a secondterminal of said first transmission line; and wherein said loopbackprevention circuit further includes a fourth FET having a grounded gate,a source of said fourth FET coupled to a gate of said third FET, a drainof said fourth FET coupled to said second terminal of said firsttransmission line.
 6. The line driver of claim 5, wherein said seconddifferential switch comprises: a fifth FET coupled between said outputof said differential amplifier and a first terminal of said secondtransmission line; and wherein said loopback prevention circuit furtherincludes a sixth FET having a grounded gate, a source of said sixth FETcoupled to a gate of said fifth FET, a drain of said sixth FET coupledto said first terminal of said second transmission line.
 7. The linedriver of claim 6, wherein said second differential switch furthercomprises: a seventh FET coupled between said output of saiddifferential amplifier and a second terminal of said second transmissionline; and wherein said loopback prevention circuit further includes aneighth FET having a grounded gate, a source of said eighth FET coupledto a gate of said seventh FET, a drain of said seventh FET coupled tosaid second terminal of said second transmission line.
 8. The linedriver of claim 4, wherein said loopback prevention circuit isconfigured to prevent said first FET from conducting when a negativepulse is incident on said first transmission line during said power downmode.
 9. The line driver of claim 8, wherein said loopback preventioncircuit is configured to apply said negative pulse to said gate of saidfirst FET.
 10. The line driver of claim 1, wherein said differentialamplifier comprises: a first FET, a source of said first FET coupled toa bias voltage, a gate of said first FET coupled to a bias controlsignal; a second FET, a source of said second FET coupled to a drain ofsaid first FET, a gate of said second FET coupled to said input signal,a drain of said second FET coupled to said first differential switch andsaid second differential switch; and a third FET, a source of said thirdFET coupled to said drain of said first FET, a gate of said third FETcoupled to said input signal, a drain of said third FET coupled to saidfirst differential switch and said second differential switch.
 11. Theline driver of claim 10, wherein said first differential switchcomprises: a first switch, having an input coupled to said drain of saidsecond FET and an output coupled to first of said first of said twotransmission line; and a second switch, having an input coupled to saiddrain of said third FET and an output coupled to said second of saidfirst of said two transmission line; wherein said first and said secondswitches open and close in response to said first control signal. 12.The line driver of claim 11, wherein said second differential switchcomprises: a third switch, having an input coupled to said drain of saidsecond FET and an output coupled to first of said second of said twotransmission line; and a fourth switch, having an input coupled to saiddrain of said third FET and an output coupled to said second of saidsecond of said two transmission line; wherein said third and said fourthswitches open and close in response to said second control signal. 13.The line driver of claim 12, wherein: said first switch comprises afourth FET; said second switch comprises a fifth FET, said third switchcomprises a sixth FET; and said fourth switch comprises a seventh FET;wherein a gate of said fourth FET and a gate of said fifth FET arecontrolled by said first control signal; and a gate of said sixth FETand a gate of said seventh FET are controlled by said second controlsignal.
 14. The line driver of claim 13, further comprising: an eighthFET, having a source coupled to said gate of said fourth FET, a gatecoupled to a ground and a drain coupled to said drain of said fourthFET; a ninth FET, having a source coupled to said gate of said secondFET, a gate coupled to said ground and a drain coupled to said drain ofsaid second FET; a tenth FET, having a source coupled to said gate ofsaid first FET, a gate coupled to said ground and a drain coupled tosaid drain of said first FET; and an eleventh FET, having a sourcecoupled to said gate of said third FET, a gate coupled to said groundand a drain coupled to said drain of said third FET, wherein saideighth, ninth, tenth, and eleventh FETs prevent signal feedthrough fromlooping back when NIC power is secured.
 15. A line driver, comprising:differential amplifier for differentially amplifying an input signal toproduce a differential amplified signal; selective coupling means forselectively coupling said differential amplified signal to one of twotransmissions lines; and loopback prevention means for preventing signalloopback from one of said two transmission lines to the other whencircuit power is removed.
 16. A line driver, for driving one of twotransmission lines, comprising: a differential amplifier that receivesan input signal; a first differential switch having an input coupled toan output of said differential amplifier and an output coupled to afirst of said two transmission lines, said first differential switchincluding, a first FET coupled between said output of said differentialamplifier and a first terminal of said first transmission line; and asecond FET having a grounded gate, a source of said second FET coupledto a gate of said first FET, a drain of said second FET coupled to saidfirst terminal of said first transmission line; a second differentialswitch having an input coupled to said output of said differentialamplifier and an output coupled to a second of said two transmissionlines, said second differential switch including, a third FET coupledbetween said output of said differential amplifier and a first terminalof said second transmission line; and a fourth FET having a groundedgate, a source of said fourth FET coupled to a gate of said third FET, adrain of said fourth FET coupled to said first terminal of said secondtransmission line.
 17. The line driver of claim 16, wherein said secondFET prevents said first FET from conducting when a negative pulse isreceived over said first transmission line during a power down mode. 18.The line driver of claim 16, wherein said fourth FET prevents said thirdFET from conducting when a negative pulse is received over said secondtransmission line during a power down mode.
 19. The line driver of claim16, wherein said first differential switch further includes: a fifth FETcoupled between said output of said differential amplifier and a secondterminal of said first transmission line; and a sixth FET having agrounded gate, a source of said sixth FET coupled to a gate of saidfifth FET, a drain of said sixth FET coupled to said second terminal ofsaid first transmission line.
 20. The line driver of claim 19, whereinsaid second differential switch further includes: a seventh FET coupledbetween said output of said differential amplifier and a second terminalof said second transmission line; and an eighth FET having a groundedgate, a source of said eighth FET coupled to a gate of said seventh FET,a drain of said eighth FET coupled to said second terminal of saidsecond transmission line.